Fragmentation of valence electronic states of CHF2CF3+ studied by threshold photoelectron-photoion coincidence (TPEPICO) techniques in the photon energy range 12-25 eV

Vacuum ultraviolet synchrotron radiation and threshold photoelectron-photoion coincidence (TPEPICO) spectroscopy have been used to study the decay dynamics of the valence electronic states of CHF2CF3+. The threshold photoelectron spectrum (TPES) and ion yield curves of the observed fragments have been recorded in the photon energy range 12-25 eV. Electrons and ions are detected by threshold electron analysis and time-of-flight (TOF) mass spectrometry, respectively. Using a combination of the measured TPES and ab initio molecular orbital calculations, we conclude that the CHF2CF3+ cation adopts a staggered C-S geometry, the (X) over tilde (2)A' ground state being formed by electron removal from the 18a' sigma-bonding orbital of CHF2CF3. Upon ionisation, large geometry changes and broad spectral bands are both predicted and observed. The next outer-valence orbitals of CHF2CF3, 17a' and l la", are predominantly associated with fluorine 2p orbitals located on the CHF2 group. Translational kinetic energy releases are determined from fixed-energy TPEPICO-TOF spectra. The ground state of CHF2CF3+ dissociates through C-C bond cleavage with a relatively small release of energy. By contrast the (A) over tilde and (B) over tilde states dissociate rapidly by an impulsive mechanism with a larger fractional release of energy to CHFCF3+ + F. Upper limits for the ionisation threshold of CHF2CF3 (12.70 +/- 0.05 eV) and the enthalpy of formation of CHFCF3+ at 298 K (299 +/- 7 kJ mol(-1)) are determined. (C) 2002 Elsevier Science B.V. All rights reserved.